1. Field of the Invention
This invention concerns the recovery of dissolved silver from solutions having been employed for the processing of silver-containing photosensitive materials.
2. Description of the Prior Art
Photosensitive materials such as films and papers employed for black-and-white photography generally have an emulsion layer which contains a finely divided silver halide such as the chloride or bromide. In the course of the development and fixing of the image within the emulsion layer, molecules of silver halide which are not converted to metallic silver by the developer are dissolved out of the emulsion layer by a "hypo" solution which generally employs a thiosulfate.
Following a certain amount of use, the hypo solution becomes spent, and must be discarded. Although it is desirable to recover the dissolved silver for its economic value, it is more important to recover the silver because it is highly toxic, and should not be indiscriminately dumped into a convenient sewer line. The United States Environmental Protection Agency has indicated that industrial effluents should contain less than 5 parts per million (ppm) silver, and drinking water should contain less than 0.05 ppm silver. Techniques generally employed for removing silver from spent hypo include electrodeposition, and treatment with a sacrificial metal such as steel wool. Hypo solutions emergent from an electrolytic cell which removes silver by electrodeposition upon electrodes will have silver concentrations between about 50 and 200 ppm. Hypo solutions emergent from steel wool systems will have unpredictably high levels of silver, depending upon the state of exhaustion of the steel wool, and will contain undesirably high levels of dissolved iron.
Attempts have been made to utilize ion exchange resins to remove the silver from spent hypo. Because the dissolved silver is in an anionic form such as silver thiosulfate Ag(S.sub.2 O.sub.3).sub.2.sup.-3, an anion exchange resin must be employed. However, ordinary anion exchange resins are not sufficiently selective to discriminate between the silver thiosulfate anion and unreacted thiosulfate and sulfite anions present in the hypo solution.
U.S. patent application Ser. No. 07/808,884, filed Dec. 18, 1991, now U.S. Pat. No. 5,180 concerns a specialized chelation-type ion exchange resin having the ability to selectively remove silver thiosulfate from spent hypo solutions. Although a worthwhile accomplishment, such removal of silver from hypo solutions would be even more practical if the silver could be eluted from the resin, permitting repeated recycling of the resin. Such elution and recycling is not disclosed in said Patent Application.
The usually employed procedure for eluting absorbed species from an anion exchange resin and re-using the resin involves treatment of the resin with strong solutions of anions that can replace the absorbed anion. In the case of the aforesaid chelation-type resins, especially when disposed within an open-celled sponge, treatment with strong solutions of the usual anions has bee-,i found to be poorly effective in eluting absorbed silver.
In the several aforesaid techniques for removing silver from photoprocessing effluents, the efficiency of silver removal is generally inversely dependent upon the rate of flow of the effluent. The efficiency of silver removal is also dependent upon the degree of saturation of whatever device or material is employed.
It is accordingly an object of the present invention to provide a process for removing silver from photoprocessing hypo solution.
It is a further object of this invention to provide a process as in the foregoing object employing a chelation-type ion exchange resin which removes said silver by selective absorption.
It is another object of the present invention to provide a process of the aforesaid nature wherein the silver absorbed on said resin can be eluted, and the resin can be re-used in said process.
It is a still further object of this invention to provide a process of the aforesaid nature wherein the efficiency of silver removal has reduced dependency upon the flow rate of said hypo solution and the degree of saturation of said resin with silver.
It is yet another object of the present invention to provide a process of the aforesaid nature which involves low capital investment and low operating costs.
These and other beneficial objects and advantages will be apparent from the following description.